Data loss due to media failure is a common concern, because storage media such as disk drives fail with alarming frequency. For example, one study of large data centers found that each year, 8-10% of all disk drives failed. Prediction of disk failures is very imprecise. Therefore, preemptive maintenance or replacement is neither certain to deter failures, nor cost-effective. The best way to safeguard data has been backup and restore operations.
Backup operations take a long time, sometimes several hours to complete. Even a single terabyte disk drive being backed up at a rate of 100 MB/sec takes about 3 hours to complete. Modern disk drives are often measured in multiple terabytes (TB) or more. For example, some modern applications collect multi-petabyte databases (e.g., years of online browsing or shopping behavior).